I have an older MC 60 with the bias circuit taken off the tube rectifiers and not the separate winding.
To start out, bias diodes and most the electrolytics (except the cans) have been replaced some time ago but I did notice the bias cap lead was disconnected so I had to re-solder.
I have a good strong -430volts off the bias circuit, once it goes thru the dropper resistor, diode and R24,25 it drops down to about -31 volts (should be 46-50)at the grid of the 6550's (also tells me both resistors are equal in value). Am I pulling grid current? The high voltage supply also drops to about 385volts so I'm guessing something in the circuit is loading the tubes.
The good amp has 435volts at the HV supply and -46 at the bias. The high voltage on the bad amp looks OK until the 6550's heat up and starts dropping.
I swapped all tubes to see if it followed a tube and it did not.
I'm a single ended guy and need some help with this.
http://www.tubebooks.org/file_downloads/McIntosh/MC60_sch.pdf
To start out, bias diodes and most the electrolytics (except the cans) have been replaced some time ago but I did notice the bias cap lead was disconnected so I had to re-solder.
I have a good strong -430volts off the bias circuit, once it goes thru the dropper resistor, diode and R24,25 it drops down to about -31 volts (should be 46-50)at the grid of the 6550's (also tells me both resistors are equal in value). Am I pulling grid current? The high voltage supply also drops to about 385volts so I'm guessing something in the circuit is loading the tubes.
The good amp has 435volts at the HV supply and -46 at the bias. The high voltage on the bad amp looks OK until the 6550's heat up and starts dropping.
I swapped all tubes to see if it followed a tube and it did not.
I'm a single ended guy and need some help with this.
http://www.tubebooks.org/file_downloads/McIntosh/MC60_sch.pdf
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The main B+ is dropping due to excessive plate current in the output tubes, which is due to insufficient negative grid bias.
Find the cause of that, and the amp should work normally. Maybe a leaking coupling capacitor or a bad R26 (the best bet)?
Is the voltage on C13 correct?
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Voltage at C13 matches that of the other amp (-435v)
R26 is fine since this just fine tunes the bias at the high side of circuit. R24,25 are the droppers and they both work equally.
Unless both coupling caps are bad wouldn't I be able to read this voltage across the coupling cap at C9,10? Pin 2,7 at 12ax7 are low bias just like the grid of 6550's
R26 is fine since this just fine tunes the bias at the high side of circuit. R24,25 are the droppers and they both work equally.
Unless both coupling caps are bad wouldn't I be able to read this voltage across the coupling cap at C9,10? Pin 2,7 at 12ax7 are low bias just like the grid of 6550's
The bias winding fails far too often in fine vintage amps, like "Macs" and "Deuces". Your MC60s are so old that a bias winding is not present. IMO, the answer to both issues is identical. A small Amveco toroid fits into the units easily enough and allows a reliable bias supply to be energized.
Talk this option over with Jim McShane and Mike Samra. They have lots of experience with implementation.
Talk this option over with Jim McShane and Mike Samra. They have lots of experience with implementation.
See my original post (and schematic), this amp takes the bias voltage off the rectifier tubes via a 1.8-5k resistor to a diode (and a cap) before it hits the two dropper resistors R24,R25. The voltage after the diode is about -430volts and can be fine tuned with the resistor just before the diode. The other amp has the same voltage at the diode (-430v) so I know the bias power supply is good. It's after the droppers R24,R25 where the voltages are wrong (-31v). As Rayma pointed out, this is biasing the tube high enough where they are drawing excessive plate current and pulling down the plate voltage below 400v.
I one of the caps were leaky wouldn't it just effect one tube (unless both are bad)?
Bad bias winding or no bias winding, adding the toroid and a few parts guarantees you have a superior bias supply.
In any event, contact Mike Samra. He's expert in refurbishing "Mac" tube amps.
In any event, contact Mike Samra. He's expert in refurbishing "Mac" tube amps.
Days ago I have finished a revamp of a MC30 pair, the schematic is quite similar.
What you can do is, with the power tubes out, read the ohms on pin 8 to ground and pin 5or 7 to ground on the Output octal socket.
When you put the tubes in you can read the voltage at these pins so you will know the current that flows in the power tubes to look if one of them is wrong; in Mc30 the values are about 13 to 16 ohms.
It is possible you have also one first HT capacitor that draw current.
Walter
What you can do is, with the power tubes out, read the ohms on pin 8 to ground and pin 5or 7 to ground on the Output octal socket.
When you put the tubes in you can read the voltage at these pins so you will know the current that flows in the power tubes to look if one of them is wrong; in Mc30 the values are about 13 to 16 ohms.
It is possible you have also one first HT capacitor that draw current.
Walter
Troubleshooting DC coupled stages can be tricky. This amp has cathode followers directly driving the output grids.
Maybe one of the coupling caps preceding those drivers is leaky. You could even disconnect both of the coupling caps C9/10
temporarily to see if the DC conditions are corrected.
Do check R26 carefully, since this dissipates power and could have changed value.
I already unsoldered one side of R26 and clipped in another in its place to check. There's not a lot of voltage drop with 430v, I played with the range from 1.8K to 4.7K and it didn't change much. I believe most people change these to fine tune the final bias around -46v.
I'm beginning to think either both coupling caps are bad or the power supply cap is pulling down the supply voltage (I did clip in a 50uf cap to check).
When I pull out the 12au7, all the voltages raise close to normal (except for the bias voltage since its unloaded).
I'm beginning to think either both coupling caps are bad or the power supply cap is pulling down the supply voltage (I did clip in a 50uf cap to check).
When I pull out the 12au7, all the voltages raise close to normal (except for the bias voltage since its unloaded).
Ok, try unsoldering one end and lifting it up for ALL of the coupling caps
C7,8,9,10. If this restores proper DC operation, replace all the coupling caps.
This might be a stupid question and I am in no way trying to be offensive. But sometimes it's something silly that is the problem. Is the polarity of C13 correct? The positive lead should go to ground. If it was wired backwards it could have failed.
Okay I see that now in post #3.
So he is referring to the cathode resistors on the followers (R24,R25) as dropping resistors. Isn't the reason he is seeing the cathodes more positive is because of the lower plate voltage the follower is getting?
So he is referring to the cathode resistors on the followers (R24,R25) as dropping resistors. Isn't the reason he is seeing the cathodes more positive is because of the lower plate voltage the follower is getting?
Upon startup, the HV is close to spec until the rectifiers warm up and the bias kicks in. It starts at about 430v and settles around 385v. It must of been this way for quite some time since some of the potting in the power transformer has melted and worked its way thru the wire holes. I think that's a good sign that the plate current is running higher than normal and overheating the tranny. From what I read, McIntosh had these running under 40ma?
Yes, referring R24,R25 as dropping resistors (or 12ax7 cathode resistor). Couldn't the cathodes be more positive from a C9,C10 leaky cap? If leaking, wouldn't the grid start pulling current and pull down the bias? That would result in dropping plate voltage? Both sides of the 12ax7 measure the same so I didn't think the caps were leaky (unless both of them are leaking). R19,21,22,23 were changed at one point during its life and measure within tolerances.
I did check that cap but you know a reversed cap wouldn't last too long at -430v.....
Yes, referring R24,R25 as dropping resistors (or 12ax7 cathode resistor). Couldn't the cathodes be more positive from a C9,C10 leaky cap? If leaking, wouldn't the grid start pulling current and pull down the bias? That would result in dropping plate voltage? Both sides of the 12ax7 measure the same so I didn't think the caps were leaky (unless both of them are leaking). R19,21,22,23 were changed at one point during its life and measure within tolerances.
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Hahaha true🙂 I had to ask though.
Now thinking about it the lower anode voltage would probably make for an increase in negative voltage. Have you lifted the coupling caps as Rayma has suggested? Are they leaky?
Now thinking about it the lower anode voltage would probably make for an increase in negative voltage. Have you lifted the coupling caps as Rayma has suggested? Are they leaky?
What are you reading in reference to ground from pins 2 and 7 of the followers? Are you reading around -55 between R22 and R19?
Here's what I don't understand. Everybody says that McIntosh amps were biased cold, but that's not what I see when I look at the voltages in the schematic.
On a Unity-Coupled output stage, the plate and screen are always at the same voltage at DC idle conditions. Therefore, we should be able to read triode characteristics to determine DC bias conditions. If I look at a 6550 data sheet, at 430V and -45V on the grid, I get about 80-85mA. This represents 36 or so Watts at idle.
That doesn't seem like particularly cold bias conditions to me.
I'm beginning to think this amp has multiple issues and needs several caps.
I lifted C9,C10 and the voltage came up from -30 to -31/32.
R22 reads around -35
If I'm reading this, with -31 volts and 385v B+, that's about 160ma or more.
That's over 60watts on the 6550, its no wonder the tubes are getting hot!
Have you tried paralleling a resistor with R22 to increase bias voltage? A 3M3 resistor paralleled with the 820k should get you -12 more volts.
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